My '49 gets a gas smell in the cab when the tank is near full also. It took me a while to figure out what was going on, but I finally did. I had pulled the seat back out and looked all around the tank and filler, trying to find a leak, with no success. Finally, I filled the tank and rocked the truck from side to side, while watching the tank. Sure enough, a wet spot started spreading out from under one of the mounting straps. It turns out that my tank has a small crack on top, directly under that steel strap. You might give the full tank/rocking test a try, it worked for me.
